1. Field of the Invention
The present invention relates to an image forming apparatus for forming image information on an image bearing member using laser beams from a plurality of laser optical sources. More particularly, the invention relates to an image forming apparatus, such as a digital copier, a laser-beam printer, a facsimile apparatus or the like, in which laser beams subjected to optical modulation from a plurality of laser light sources are guided onto an image bearing member, such as a photosensitive member, an electrostatic recording medium or the like, and image information comprising, for example, an electrostatic latent image is formed on the surface of the image bearing member.
2. Description of the Related Art
Conventionally, in a laser driving circuit of image forming apparatuses of this type, in order to protect a laser from being destructed, the output of the laser beam is monitored, and the driving current for the laser is limited or interrupted when the output exceeds a specified value.
A specific control method in the above-described approach will now be described with reference to FIG. 1. FIG. 1 is a block diagram illustrating the configuration of a conventional 1-beam laser driving circuit. In FIG. 1, there are shown a laser chip 510, a laser 511, a PD (photodiode) sensor 513, a pulse current source 521, a bias current source 523, a switch 531 for a pulse, a switch 533 for the laser, a PWM (pulse-width modulation) circuit 535, a current-to-voltage converter 541, an amplifier 542, a sequence controller 543, a comparator 545 and an APC (automatic power control) circuit 550.
Conventionally, in an image forming apparatus of this type, as shown in FIG. 1, the laser chip 510 comprising the single laser 511 and the PD sensor 513 is used. By using the two current sources, i.e., the bias current source 523 and the pulse current source 521, for the laser 511, the emission characteristics of the laser 511 are improved. In order to stabilize the emission of the laser 511, the amount of a bias current is automatically controlled by effecting feed-back to the bias current source 523 using an output signal from the PD sensor 513. That is, the output signal from the PD sensor 513 is input to the current-to-voltage converter 541, and is then amplified by the amplifier 542. The output of the amplifier 542 is input to the APC circuit 550, and is then supplied to the bias current source 523 as a control signal. This circuitry is called APC circuitry, which is now generally used as circuitry for driving a laser. The laser has a temperature characteristic such that as the temperature is higher, the amount of current for obtaining a constant amount of light is larger. In addition, since the laser performs self-heating, a constant amount of light cannot be obtained only by supplying a constant current. This phenomenon greatly influences image formation. In order to solve such a problem, the above-described APC circuirtry has been devised. By adopting this circuitry, it is possible to obtain a constant amount of light without being influenced by the ambient temperature of the laser, and to inexpensively realize appropriate image formation. The output of the PD sensor 513 is used not only for stabilizing the output of the laser, but also for protecting the laser from being destructed. The laser is very easily destructed. If the laser is used in a condition slightly exceeding a rating, it will be degraded or destructed in a short time. In a circuit for protecting the laser, a limiting value LIM which is slightly larger than a reference value for an optical output usually used in image formation is set. As shown in FIG. 1, the output of the PD sensor 513 is compared with the limiting value LIM using the comparator 545. When the output of the PD sensor 513 exceeds the limiting value LIM, a warning signal indicating abnormal emission is output. While the warning signal is output, current supply to the laser is limited or interrupted using the switch 533 for the laser comprising a transistor switch or the like.
In digital image forming apparatuses, there is a strong demand to increase an image forming speed. In order to reply to such a demand, image forming apparatuses in which a laser driving circuit using a multiple lasers is provided have been proposed. The above-described protection circuit is also indespensable in such an apparatus, and it is requested to protect degradation or destruction of a plurality of lasers. In this case, a configuration as shown in FIG. 2 which is similar to the configuration shown in FIG. 1 is used.
FIG. 2 is a block diagram illustrating the configuration of a conventional twin-beam laser driving circuit. In FIG. 2, there are shown a multilaser chip 610, an A laser 611, a B laser 612, a PD sensor 613, an A pulse current source 621, a B pulse current source 622, an A bias current source 623, a B bias current source 625, a switch 631 for an A pulse, a switch 632 for a B pulse, a switch 633 for the A laser, a switch 634 for the B laser, a PWM circuit 635 for the A laser, a PWM circuit 636 for the B laser, a current-to-voltage converter 641, an amplifier 642, a sequence controller 643, a comparator 645, and a time-division APC circuit 650.
In FIG. 2, the switch 631 for the A pulse and the switch 632 for the B pulse switch on/off the A pulse current source 621 and the B pulse current source 622 in accordance with an output signal from the PWM circuit 635 for the A laser and an output signal from the output from the PWM circuit 636 for the B laser, respectively; The amplifier 642 amplifies the output of the PD sensor 613 converted into a voltage by the current-to-voltage converter 641. The comparator 645 compares the output of the PD sensor 613 converted into the voltage and amplified by the amplifier 642 with the limiting value LIM for the optical output, and outputs a warning signal indicating abnormal emission when the output of the PD sensor 613 exceeds the limiting value LIM. While the warning signal is output, current supply to the A laser 611 and the B laser 612 is limited or interrupted using the switch 633 for the A laser and the switch 634 for the B laser, each comprising a transistor switch or the like, respectively.
However, in a multilaser configuration in which a plurality of lasers are provided in a chip, a single PD sensor is provided commonly for the plurality of lasers because of restriction in the configuration of the chip. Hence, in the above-described conventional protection circuit, only the sum of optical detection signals for the plurality of lasers is known when, for example, both the A laser and the B laser are simultaneously lit. As a result, even if the optical output of one laser exceeds the limiting value, current supply for that laser is not limited or interrupted unless the sum of the optical outputs of the lasers exceeds the sum of the limiting values. Furthermore, it is impossible to specify the laser which is degraded or destructed.